A diaphragm type silicon based pressure sensor will typically include piezoresistors positioned to sense strain associated with pressure and arranged in a Wheatstone bridge to which a direct current voltage is applied. The output voltage of the bridge is representative of the pressure that is being sensed. When no pressure is sensed the output of the bridge should be zero or null. However, slight differences in the bridge resistors or other causes will typically produce some initial offset from null upon power up of the bridge. Thus, a power up drift (PUD) phenomena has been observed in silicon based sensors (that is not explained by a small thermal rise) that may occur after power is applied to the sensor.
The PUD phenomena is apparently a result of charges (e.g., mobiles ions) present in a silicon chip or on the surface of a silicon chip, which have one preferred configuration with power off and a second preferred configuration with power on. That is, the charges move in response to the application of voltage to the silicon chip. As the charges move they apparently affect the characteristics of the circuit elements on the chip. The charges may reside in any of a number of locations in the integrated circuit. The charges may be in the silicon, in insulating layers on or under the silicon, at the interfaces between two of these layers, or at the surface of the silicon chip, for example. For more information regarding PUD, the reader is referred to U.S. Pat. No. 6,065,346, the contents of which are herein incorporated by reference, as if fully set forth in this description.
In some pressure sensors, the charges from a PUD can be larger than the voltage output changes that are related to pressure readings. The PUD phenomena is typically of little consequence for digital circuitry as the change in charge location usually results in voltage changes that are much smaller than the rail voltages used. However in some circuitry, including pressure sensor circuitry, a bridge configuration is designed to minimize changes in power up voltages, and other performance limitations.
In a bridge configuration, the change of any one element resulting from redistribution of charges on power up may not be significant as long as the bridge's balancing element undergoes the same change. Therefore, care is usually taken in the design of a sensor to insure that the individual elements of the bridge are as identical as possible. As a result, the power-up drift of the bridge output “resets” itself after the power is removed to the value that existed before power was applied.
Within pressure sensors that employ a conventional full (4-arm) Wheatstone bridge mechanization, powered by a constant voltage source, a differential voltage output proportional to a pressure can be sensed. High performance sensors can also include an on-chip full Wheatstone temperature bridge. The temperature output can then be used to compensate and calibrate the pressure output using microprocessor-based electronics, for example. Thus, high performance sensors including both pressure and temperature Wheatstone bridge mechanisms include two separate configurations on the same chip.
However, full bridge mechanizations including pressure and temperature Wheatstone bridge mechanisms, can be susceptible to non-compensatable errors such as non-ratiometricity errors, power-up drift, thermal hysteresis, and time dependant high temperature induced drift (HTNR). These errors may be related to one or more of the following: a difference in voltage sensitivity of elements in a top of the Wheatstone bridge compared to those in the bottom of the Wheatstone bridge, migration of ionic contaminants in the presence or absence of an electrical field, and the magnitude of the sensor voltage source.
In addition, numerous applications for high performance sensors may require high accuracy in the sensor's outputs, and the constant voltage source of the sensor can cause charges from a PUD to alter the pressure sensor's voltage output resulting in unacceptable readings. Further, existing high performance sensors require complex configurations to enable both pressure and temperature sensing. Thus, a less complex sensor not susceptible to PUD effects is desirable.